Performance analysis of the two spectroscopic imaging sequences LRE and EPSI

Introduction: The practical value of high spatial resolution chemical shift imaging (CSI) is compromised by long total scan durations. To achieve shorter measurement times, fast pulse sequences for spectroscopic imaging (SI) can be employed at the expense of lower sensitivity relative to conventional CSI (1). Based on a gradient balanced free precession sequence Linear Response Equilibrium (LRE) has been shown to be a promising method for spectroscopic imaging with high sensitivity as coherent transverse magnetization contributes to the periodically evolving steady-state signal (2). In LRE varying RF pulse amplitudes, modulated by a periodic flip function ω(t), allow for the design of a desired excitation profile in the frequency domain. The feasibility of LRE for fast spectroscopic imaging with simultaneous spatial and spectral encoding and an intrinsic water or fat suppression has been demonstrated previously (3). On the other hand, Echo Planar Spectroscopic Imaging (EPSI) has been widely used as it provides the same high signal-to-noise ratio per unit time (SNRt) as classical CSI (1) and also permits simultaneous spatial and spectral encoding. In this work, we compare the fast spectroscopic imaging sequences LRE and EPSI in terms of sensitivity. The signal of both methods was calculated numerically and compared in controlled phantom experiments.